Healthy and normal nonhuman primates are required for many biomedical research studies. Although guidelines and regulations exist for the maintenance of healthy nonhuman primates which include standards for nutrition, sanitation, and the micro- and macro-environments, the 1985 amendment to the Animal Welfare Act (PL 99-198) mandates additional requirements with the introduction of the term "psychological well-being." Many means of measuring "psychological well-being" are currently used. The most commonly utilized measures include behavioral and physiological parameters. Abnormal behaviors frequently seen in rhesus monkeys maintained in laboratories have described in great detail. Similar descriptions for other nonhuman primate species are also available. Physiological measures, such as cortisol and ACTH levels as well as immune status tests (e.g., IgG levels) are objective values that can be monitored at regular intervals. However, the validity of relying on measures of stress as the sole determination of psychological well-being has not been verified. Also, the ubiquitous usefulness and reliability of any measure for even one species of nonhuman primates is unlikely as individual variation in situational responsiveness is high in nonhuman primates.